1. Field of the Invention
This invention relates to curable coating compositions. More particularly, this invention relates to a class of thixotropic coating compositions containing at least one polyfunctional epoxide reactant and at least one organosilicon reactant. The amount of thixotropic character exhibited by the compositions, i.e. the extent to which they flow under zero shear, is controllable when the weight ratio of the two types of reactants is within specified limits. The compositions are particularly useful as extrudable coating compositions for solid state electronic devices.
2. Description of the Prior Art
Solid state electronic devices such as transistors and integrated circuits are typically very delicate and must be protected against physical damage and atmospheric moisture. Such protection is often achieved by encapsulating the device in a hard, chemically resistant material such as an epoxy resin or a copolymer of an epoxy resin and an organosilicon reactant.
One method for encapsulating electronic devices employs a technique known as transfer molding. The device is placed in a mold and a quantity of molten resin sufficient to fill the mold cavity is injected and allowed to solidify around the device, leaving the electrical connections or leads to the device exposed.
Transfer molding has a number of disadvantages, including the cost of encapsulating a number of different devices individually, prior to mounting them on a circuit board. In addition, it is sometimes difficult to achieve an adequate moisture proof seal around the leads which project through the encapsulating material and serve to connect the device to the circuit board. A more economically advantageous alternative to transfer molding is to mount a number of solid state devices on a circuit board and encapsulate the devices in a single operation. Because it is usually not practical to fabricate a mold to accommodate an entire circuit board, the composition used to encapsulate the devices should be sufficiently thixotropic that a coating can be applied by extruding the composition on to the device under relatively low pressure. Once in place the coating should flow only to the extent that it will form a smooth surface without otherwise altering its original contour. This property must be exhibited both at ambient temperature and the temperature at which the coating is cured, which can be up to 200.degree. C.
The use of copolymers derived from polyfunctional epoxide and organosilicon reactants for encapsulating electronic devices is disclosed in the prior art. For example, U.S. Pat. No. 3,971,747, which issued to Bank and Michael on July 27, 1976, teaches that molding compositions suitable for encapsulating integrated circuits by transfer molding contain copolymers derived from a "phenylmethylsiloxane resin" and commercially available epoxy resins. The copolymers are prepared in the presence of specified aluminum-containing catalysts. U.S. Pat. No. 4,082,719, issued to Liles and Michael on Apr. 4, 1978, teaches that the addition of a small amount of an organosilicon material containing at least one silicon-bonded hydrogen atom per molecule to the reactants disclosed by Bank and Michael improves the performance of the epoxy-organosilicon copolymers as encapsulating materials for electronic devices. In particular, the tendency of water to "wick" up exposed leads of the devices is reduced.
U.S. Pat. No. 3,842,141, which issued on Oct. 15, 1974 to Fetscher and Sparapany discloses molding compositions prepared from a specified class of epoxy-organosilicon copolymers. The compositions exhibit desirable properties such as high volume resistivity, low water absorption and good hot strength, and are used to encapsulate integrated circuits by conventional molding techniques.
None of the aforementioned patents teach employing the disclosed copolymers or precursors thereof as ingredients of extrudable compositions that can be coated onto integrated circuits or other devices mounted on a circuit board at ambient temperature and subsequently cured without any substantial change in configuration of the initial coating.
It is known to impart thixotropy to liquid coating compositions by adding solid or liquid thixotropic agents such as cellulose esters, however this method has a number of disadvantages. In addition to increasing the raw material cost of the composition, the interaction of the thixotropic agent and the other ingredients of the composition may undesirably modify properties of the uncured or cured compositions. This interaction may be instantaneous or gradual and could result in a complete or partial loss of desirable properties, including thixotropy, during storage of the composition. Finally, any modification of the type or proportions of the ingredients, including reactants and filler, will usually require a change in the amount and/or type of thixotropic agent to achieve acceptable performance as a protective coating composition, i.e., a smooth surface with retention of initial coating configuration.
One objective of this invention is to provide thixotropic coating compositions containing epoxide and organosilicon reactants.
Another objective of this invention is to provide means for varying the degree of thixotropy exhibited by coating compositions containing epoxide and organosilicon reactants without the addition of conventional thixotropic agents or other modifiers.